A sodium-sulfur battery is a secondary battery obtained by isolating molten metal sodium which is a cathode active substance from molten sulfur which is an anode active substance by a beta alumina solid electrolyte having selective permeability with respect to sodium ions. During discharging, molten sodium releases electrons that become sodium ions, and these ions pass through a solid electrolyte tube to move to an anode side, and react with the electrons passed through sulfur and an external circuit to generate sodium polysulfide, thereby generating a voltage of about 2 V (with an electric cell). During charging, in reverse to the discharging, a generating reaction of sodium and sulfur occurs. In an operation cycle of the sodium-sulfur battery for a use application such as leveling of a load, such a cycle including the discharging and charging repeats with intermissions.
Such a sodium-sulfur battery usually has a constitution in which a plurality of electric cells are connected in series, to form a string, a plurality of strings are connected in parallel, to form a block, a plurality of blocks are connected in series, to form a module, and a plurality of modules are further connected in series.
Moreover, for the purpose of causing a reaction of xS+2Na++2e−←→Na2Sx as described above, the sodium-sulfur battery is operated while keeping a temperature (substantially, the temperature around the module) above a melting point of sodium polysulfide, for example, in a range from about 280° C. to 350° C. (approximately 300° C.). Therefore, at the start-up of the sodium-sulfur battery for shipping, it is first necessary to increase a temperature of the battery by heaters disposed around the modules. Heretofore, at the start-up of the sodium-sulfur battery, the temperature has been increased from ordinary temperature to approximately 300° C. with a predetermined temperature gradient.
It is to be noted that examples of a prior-art document include JP-A-2004-111123.